1. Field of the Invention
The invention relates to a process for acquiring measurements with an X-ray tomodensitometer, and a tomodensitometer for implementing the process. These apparatuses generally use a single source of X-rays and a multi-element detector opposite the source. The source and detector assembly can revolve and or move in translation relative to the body of a patient who is placed between the source and the detector.
2. Discussion of the Background
In general, the X-ray detector includes a multiplicity of individual detection elements juxtaposed so as to cover the whole of a circular arc illuminated by the X-ray source. There are nevertheless detectors which are not curved. The most well known detectors are gas-based detectors and solid-state detectors. The signals output by the detector elements are gathered periodically, digitized and transmitted to a computer in tandem with the rotation, and or with the translation of the apparatus, so as to be processed by digital computation in order to reconstruct an internal image of a slice of the body exposed to the X-rays.
For greater image resolution, it is desirable to have a large number of detection elements juxtaposed along a circular arc, or more generally along a line situated in a plane substantially perpendicular to the axis of rotation translation. Moreover, the detector can be constructed in the form of several detection assemblies juxtaposed in a direction parallel to the axis of rotation translation. In this case, the individual detection elements can be arranged as several strips of two or more elements. These elements are aligned in a direction parallel to the axis of rotation, the strips being juxtaposed along the circular arc. This makes it possible in particular to simultaneously explore two or more very fine juxtaposed slices of the body examined, since the source, by revolving, simultaneously exposes two or more adjacent series of detectors of small dimension. Such an apparatus is for example described in the document U.S. Pat. No. 5,592,523.
By way of example, a tomodensitometer can use as detection elements up to several thousand photodiodes, of the polarized type or of the photovoltaic type, individually covered by a scintillator crystal, and this number could even increase to several tens of thousands. The example which will be studied here will include around 25,000 detection elements. This results in difficulties with regard to gathering the electrical signals emanating from these numerous detection elements. The aim of the invention is to propose a mode of operation allowing the best possible solution of the technical constraints resulting from the presence of this large number of detection elements.
Among these constraints, there are:
the necessity to read the signals from all the detection elements over a very short duration, for example of the order of 0.5 milliseconds. This is because a tomodensitometer revolves continuously at a speed, for example conventional, of two revolutions per second. In a preferred example, one wishes to perform around 1000 views of the body for each revolution. A view corresponds to a given duration in the course of which the body is exposed to X-ray radiation, in a continuous or pulsed manner. The image of the view revealed on the detector corresponds to the phenomenon of radiological absorption which occurs for the duration of the view. One distinguishes the image from the view itself (portrayed directly in projection mode by the detector), and the reconstructed image of a body section in which this view participates in the reconstruction calculations. For the duration of a view, the tomodensitometer is regarded as occupying a fixed position relative to the body when processing the image. Of course, this is not true since the tomodensitometer is moving continuously. A view is nevertheless thus associated with an angle of incidence, a position: the tagging of the mean angle at which a principal axis of X-ray radiation irradiates the body. If the duration of measurement, the duration of view, were greater the final resolution of the image would not be acceptable: it would then be necessary either to take fewer views, or rotate the tomodensitometer less quickly;
the necessity to read signals over a very large dynamic swing, for example of the order of 20 bits, since the dark parts of the image yield an extremely weak signal relative to the lighter parts, and this weak signal must however be measured with some resolution (a few bits). Thus, in order to measure both 1,000,000 X-ray photons received in nominal mode, or 4 X-ray photons received at minimum, a measurement dynamic swing of 20 bits would be necessary. However, if the overall dynamic swing of the image thus represents 20 bits, the useful local dynamic swing is less demanding. This is because this useful local dynamic swing is equal to the signal-to-noise ratio. In actual fact, the signal-to-noise ratio is equal to around 1000 maximum. It is in fact equal to the square root of the number of X-ray photons received owing to the quantum phenomenon of absorption of X-rays in the body and their conversion in a scintillator. This means that out of the 20 bits measured, only 14 bits are significant. The digitizing of the measurements over such a dynamic swing nevertheless involves the adoption of circuits which are overdimensioned in terms of performance and occupy too much room in an integrated circuit;
in a solid-state detector, the necessity to process quantities of charge which may be fairly high when the detector receives large fluxes of X-ray radiation. Nominal charges of the order of 100 picocoulombs, pC, per photo diode are in fact encountered. Such charges require large storage areas. This, however, poses size problems with regard to achieving the required capacities for storing these charges, given the low operating voltages of these integrated circuits. By way of example, the detectors can be diodes of the reverse-bias type, a capacitor being fashioned at the terminals of a diode owing to its reverse bias. A preamplifier can be connected successively to each of these diodes so as to recharge the capacitor with the charges which it lost under the action of the light received by the diode. The quantity of charge reinjected by the preamplifier forms the measurement signal. Preferably, for linearity reasons, non-biased diodes, of the photovoltaic type, will be used. They produce a DC current proportional to the illumination which they receive. The measurement of the X-ray radiation is effected therein via a link from this photovoltaic diode to an integrator, and via the integration over a small duration of this current signal. At each new duration the integrator is previously reset to zero. In this latter case the integrator is the facility for storing the charges to be measured whereas in the previous case it is the reverse-fashioned capacitor.
To allow the construction of an apparatus complying with these constraints without involving a prohibitive manufacturing cost, the present invention proposes a process for obtaining tomographic images, preferably using at least one array of elementary detectors, a charge storage element associated with each detector, and a circuit, a multiplexer, associated with the array of storage elements so as to periodically instruct the connecting of the various detectors to their storage elements. The successive analogue signals output by the multiplexer are transformed into digital signals downstream by an analogue/digital converter.
The principle of the invention then consists, in the course of a view, in splitting up the signal integration time so as to reduce the quantity of charge to be measured. Indeed, under these conditions the detector facility used need no longer be capable of accumulating large quantities of charge. Moreover, the analogue/digital converter no longer needs such a large dynamic swing. It can be shown that one thus goes from 20 bits to 14 bits. Because samplings and multiple quantizations are then effected during a view, it is then of course necessary to execute a summation of the quantization results so as to produce a signal corresponding to the view, for each detector.
However, the sought-after result is attained even so, namely:
the capacity required for the storage elements is reduced by a factor n (n being a measurements acceleration factor),
the frequency of the analogue/digital converter is increased by the factor n,
the dynamic swing of this analogue/digital converter is, overall, reduced in a factor lying between n and root n.
The subject of the invention is therefore a process for acquiring measurements with a tomodensitometer comprising the following steps:
a body is irradiated, during a given view, with X-ray radiation,
an analogue signal is measured for this view, in each detector of an array of detectors, this signal representing the effect of the absorption of the X-ray radiation in the body at the location of each of these detectors,
each analogue detector signal is sampled and converted into a digital detector signal, characterized in that
the analogue signal from each detector is sampled with n repetitions in the course of the view, and
the n converted signals from each detector are added together to construct the digital view signal of each detector.
Its subject is also a tomodensitometer furnished with a device for acquiring measurements comprising
an X-ray tube for irradiating a body with X-ray radiation during successive views,
an array of detectors for measuring an analogue signal representing the effect of the absorption of the X-ray radiation in the body at the location of each of these detectors during a view,
an analogue/digital converter for sampling and converting each analogue detector signal into a digital detector signal,
a sequencer for driving the array of detectors and the converter at the rate of the views, characterized in that it includes
means in the sequencer for driving the array of detectors and the converter at a rate n times greater than the rate of the views and,
an adder for adding together the n converted signals from each detector so as to construct the digital view signal of each detector.
Another problem to be solved with such tomodensitometers is related to the number of their detectors, which as stated hereinabove may be very large. The assembly of control circuits of all these detectors then constitutes a very voluminous system to be constructed, even when employing the most modern miniaturization techniques. It is not in reality easy to go from manufacturing a detector with 700 detection elements to a detector with 25,000 detection elements.
To solve this other problem, according to another characteristic of the invention, groupings of detectors are constructed and common processing is applied to all the detectors of these groups. The invention starts in fact from the following principles which it has highlighted. Firstly, in a projection-mode image, the change in contrast is never abrupt, even if the dynamic swing of measurement over the entire image is itself large. This is because the organs of the body of a patient either interpenetrate or are viewed in projection through other organs. Therefore, there is always a transition zone between the images of these organs. This transition zone is relatively large on the scale of the size of the detectors. For this transition zone, the contrast may then be regarded as not changing excessively.
Secondly, and by way of adjunct, because of the slow rotation of the tomodensitometer, one may regard the absorption phenomenon measured in a detector, at the moment of a view, as being the same phenomenon (with the same dynamic swing) as the phenomenon which occurs in a neighbouring detector at a following view.
Stated otherwise, in the invention, it has then been considered that it was already possible to group the detectors of a region. The effect of this grouping is to subject the detectors of a group to one and the same mode of measurement. In practice, these considerations have led to the construction of groups of detectors for which the measurement dynamic swing may be regarded as lying within the same range. According to the invention, each group of detectors is then allocated a measurement range. This leads to the simplifying of the electronic detection circuits. The range is determined by ensuring that the greatest measurement performed for a detector of a group lies within the smallest possible measurement range assigned to this group. By way of improvement, the determination is performed during a view, and the assignment is carried out at the following view.